Sir, your estimates for ...

Sir, your estimates for daily consumptions per family our pretty good for master planning purposes. There are two sides as you know well - the supply and demand.  You will need to estimate population growth and multiple that with per capita consumption. For population growth, you can estimate the growth based on previous growth in the region. I think the Pakistan government or the UN shows have this data on their website. Use the current population and the growth rate to project future water needs. It is a simple excel calculation. As noted below by Dave, assume a peaking factor for increased usage as population increases and people become more prosperous.  

On the supply side, you will need to estimate what you can get from your surface sources  - rivers, lakes, etc. Usually, there is safe and sustainable limit for withdrawal from these sources established by local and national govt (but no one obeys that in our regions). If you can get flow rate of a year (average) you can estimate the volume that flows through your City. Assume you can safely use 20-40% of the volume as supply. For lakes, you can also assume a percentage of volume for use based on the rainfall or snow melt that recharges the lake. Estimation of groundwater is not easy for this effort unless you have more data. 

I believe you can show a nice story with these calculations. As you noted, the current supplies might not support future growth. Hope this helps. Goodluck!